The seismic method of delineating geological subsurfaces of the earth involves the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy generally is a high explosive charge electrically detonated in a borehole located at a selected grid point in the terrain. The acoustic waves generated in the earth by the explosion are reflected back from pronounced strata boundaries and reach the surface of the earth after varying intervals of time, depending on the distance and the nature of the subsurface traversed. These returning acoustic waves are detected by the geophones, which function to transduce such acoustic waves into representative electrical signals. The plurality of geophones are arrayed in a selected manner to detect most effectively the returning acoustic waves and generate electrical signals representative thereof from which data may be deduced concerning the geological subsurface of the earth.
Seismic geophysical exploration has been modified in recent years by some seemingly small, but significant changes. One such change relates to the signal enhancement produced by the common depth point method as illustrated in the United States letters Patent 2,732,906 to Mayne. As a result of this improvement, it has become possible to utilize sources of seismic energy which are much weaker than explosives, but which are frequently much more practical and convenient. However, in practice, the common depth point method requires very large arrays of geophones, and these weaker seismic energy sources often require the summing or vertical stacking of detected responses to a large number of individual "shots" by the same or different seismic energy sources.
It is common today in the art to utilize at least twenty-four separate groups of geophones arrayed over a distance of some two or three miles. In many seismic systems utilized today in the art, the geophones in the array are electrically connected to a central recording apparatus by means of a long multi-conductor or coaxial cable, and the electrical signals generated by the geophones are coupled through the cable to the central recording apparatus.
There are numerous problems involved in the use of long cables to bring the signals from the geophones to the central recording station: resistivity losses can be quite high; the lines tend to pick up extraneous electrical noises; the cables tend to dictate that the arrays of geophones be linear; the geophone intervals are fixed by the construction of the cables; laying out the cable itself may be a major undertaking, such as in jungles, swamp or marsh; the cables are subject to attack by animals as well as humans; the cables are subject to water infiltration and mechanical damage; the cables are expensive and require repairs, upkeep and spare parts in remote areas; the central recording apparatus tends to be bulky; and multiplexing problems associated with digital recording arise when numerous geophone groups are utilized.
There have been attempts to develop improved seismic geophysical exploration systems which, among other things, eliminate the long cables. Examples of such systems are the systems disclosed in United States letters Patent Nos. 2,749,772; 3,283,296; and 3,806,864. However, for one reason or another, each of the above prior art systems is disadvantageous. By way of example, in the prior art systems which eliminate the use of long cables, there is no means for the data produced by the geophones responsive to a particular shot of the seismic energy source to be summed or stacked by the remote monitoring units with the data produced by the geophones responsive to other shots of the seismic energy source. In addition, these prior art systems which eliminate the use of long cables require superior radio communication between the control unit and the remote monitoring units. Under noisy conditions, the time of the shot by the seismic energy source may be uncertain and thus lead to degradation of the seismic data.